1. Field of the Invention
The present invention relates to DAT (Digital Audio Tape) recording systems. More specifically, the present invention provides a method for determining a read amplitude threshold which is used to decide when data written to tape should be rewritten for reliability in a read-after-write data-checking process.
2. Description of the Related Art
Although the present invention can be incorporated into other embodiments, it is described as embodied in a helical-scan DAT recording system. As with all magnetic media, the magnetic tape used in DAT recording systems contains some tiny areas where the oxide is not as good as it is in the majority of the tape. If the oxide is sufficiently bad, the area is a media flaw. Media flaws can be caused by foreign particles in the oxide, (such as from cigarette smoke), scratches on the oxide, inclusions which are foreign, molecular-level defects in the oxide, and substrate deformities (such as might be caused by the plastic backing layer of the tape).
When data is written in a media flaw area, the amplitude of the signal reproduced by the read head is reduced and may not be sufficient to correctly reproduce the data that was written onto the tape. Thus, writing in media flaw areas can cause a loss of data. For this reason, certain precautions are taken when writing data to tape so that the data can be recovered.
One generally used technique for insuring the recovery of data that is written to tape is a read-after-write data-checking process. In such a process, data is read immediately after it is written, to determine if it is recoverable. In many systems, a dedicated read head is used so that the data is read immediately after being written. If the data is not recoverable, it is rewritten in a different location on the tape. The nonrecoverable data is usually rewritten in an area closely following the area where the error occurred.
A typical method of determining whether data is recoverable is to use a selected read amplitude threshold during the read portion of the read-after-write process. The read threshold during the read-after-write process is selected to be greater than the read threshold for conventional reading. If the just-written data produces a signal amplitude greater than the read-after-write read threshold, then the data should be recoverable when it is read at the normal read threshold. If the amplitude is below the threshold, the data may not be recoverable and should be rewritten.
Currently, many different methods are used to set the read amplitude threshold. One method is to set a threshold for a particular recording system once by writing on a sample tape and then measuring the read signal amplitude. The problem with this technique is that tapes vary in oxide quality such that nominal amplitudes can vary as much as .+-.30% from tape to tape. Thus, if a recording system has a threshold set by a tape that produces a low-amplitude data signal, the threshold will be too low for a tape that produces a higher amplitude data signal. Conversely, if the threshold is set for a sample tape that produces high-amplitude signals, the recording system may not be able to accurately reproduce tapes that produce low-amplitude signals. Even if an average tape is tested, other tapes will vary significantly with respect to the levels of signals that they produce.
A second method of setting the threshold is to write and read back a sample written on a given portion (such as the beginning) of each tape. However, there are at least two problems with this second method. One problem is that there may be some media flaws on the tape where the sample is written. If the sample is written in a media flaw area, the read-back signal will have a low amplitude. A threshold value based on this abnormally low signal will be too low, and thus will falsely indicate that signals in a media flaw area are recoverable.
A second problem with this second method is that the actual writing of data will occur at different locations on the tape. For example, different locations on the same tape may have different amounts of oxide caused by the initial manufacturing process or by differences in wear (e.g., the first portion of a tape will generally be used more than other portions of the tape). Since different amounts of oxide produce different nominal amplitudes, different locations on the same tape produce different nominal amplitudes. Therefore, different locations on the same tape will require different amplitude thresholds in order to accurately reproduce the data written thereon. For this reason, it is necessary that the threshold be set at the location on the tape where the user desires to write.
It is an object of the present invention, therefore, to provide a method for setting a threshold that will ensure the reliability of data written to a given area of the tape.